Optoelectronic devices based on nitrides are generally fabricated on sapphire or silicon carbide substrates which differ from nitride layers deposited thereon (so-called heteroepitaxy). However, qualities of such devices that have epitaxially grown on heterogeneous substrates are not sufficiecnt.
Therefore, there is a demand for a process for forming not only a bulk monocystalline GaN but also a bulk monocystalline AlN. The following are proposed as the process for forming bulk mono-crystalline GaN: Halide Vapor Phase Epitaxy (HVPE) employing halides in vapor phases [“Optical patterning of GaN films”, M. K. Kelly, O. Ambacher, Appl. Phys. Lett. 69 (12) (1996) and “Fabrication of thin-film InGaN light-emitting diode membranes” W. S. Wrong, T. Sands, Appl. Phys. Lett. 75 (10) (1999)]; the HNP process using high-pressure nitrogen [“Prospects for high-pressure crystal growth of III–V nitrides”, S. Porowski et al., Inst. Phys. Conf. Series, 137, 369 (1998)]; the ammono method using supercritical ammonia so as to lower the temperature and the pressure for a growing step [“ammono method of BN, AlN, and GaN synthesis and crystal growth” R. Dwilinski et al., Proc. EGW-3, Warsaw, Jun. 22–24, 1998, MRS Internet Journal of Nitride Semiconductor Research]; and [“Crystal Growth of gallium nitride in supercritical ammonia”, J. W. Kolis et al., J. Cryst. Growth 222, 431–434 (2001)].
On the other hand, as for bulk monocrystalline AlN, D. Peters has proposed a process for forming aluminum nitride crystals from metallic aluminum by using a supercritical ammonia (Journal of Crystal Growth 104 (1990), pp 411–418), however, the resultant crystals were very fine and only for use in packages. Recently, as an epitaxial growth method, Y. Shi et al. have succeeded in growing monocrystalline AlN on a SiC substrate through an AlN buffer layer by the sublimation method (MIJ-NSR, Vol. 6, Art. 5). However, improvement on the quality of crystals is limited, since the vapor phase growth is based on non-equilibrium chemical reaction. In the meantime, the lives of optics using semiconductors vary depending on mainly the crystallinity of the active layers, involving a dislocation density. In case of a laser diode employing an AlN substrate, it is preferable that the dislocation density thereof is decreased to 106/cm2 or lower, which is, however, very hard for the conventional processes to achieve.